Internal combustion engines of two-stroke or four-stroke type usually are equipped with a fuel supply system of carburetor type or injection type. In a carburetor, the throttle of the carburetor is affected by the operator's demand, so that a wide open throttle produces a minimum throttling in the carburetor barrel. The depression created by the passing air in the carburetor venturi draws fuel into the engine.
Diaphragm-type carburetors are particularly useful for hand held engine applications wherein the engine may be operated in substantially any orientation, including upside down. Such carburetors typically include a fuel pump that draws fuel from a fuel tank and feeds the fuel to a fuel pressure regulator via a needle valve. The fuel pressure regulator usually includes a fuel metering chamber that stores fuel fed from the fuel pump and the fuel metering chamber is generally separated from atmosphere by a diaphragm that adjusts the fuel pressure to a constant pressure. The needle valve opens and closes the fuel passage from the fuel pump to the fuel metering chamber as the diaphragm moves. From the fuel metering chamber fuel is delivered to the main air passage via a main channel and an idle channel. The main channel leads to a main nozzle in the main air passage fluidly prior to the throttle valve, whereas the idle channel leads to an idle nozzle fluidly shortly after the throttle valve.
When starting a crankcase-scavenged engine having a conventional carburetor, the choke valve is closed by the operator using a choke button and the throttle valve is set in a start gas position. When pulling the pulling cord to start the engine, an air and fuel mixture is delivered to the crankcase of the engine. When a first ignition is heard by the operator, the choke valve is opened so not to flood the engine with too much fuel. However, sometimes the operator misses the first ignition and the engine is flooded and the product cannot be started as desired.
U.S. Pat. No. 6,932,058 discloses a carburetor including a fuel supply system for supplying fuel from a diaphragm controlled regulating chamber to the intake channel of the carburetor. The fuel supply system includes a main fuel path having a control valve and an idling fuel path that branches off from the main fuel path downstream of the control valve. The control valve thereby controls all fuel supplied to the intake channel. It has however been found out that this solution provides an inadequate fuel supply in certain situations. In particular it is difficult to control the fuel supply at start up.
U.S. Pat. No. 7,603,983 shows a carburetor including a fuel supply system having two independent fuel paths for supplying fuel from a diaphragm controlled regulating chamber to the intake channel of the carburetor. The first fuel path includes a main fuel path having a control valve and an idling fuel path that branches off from the main fuel path downstream of the control valve. A first bypass line bypasses the control valve. The second fuel path connects the regulating chamber to an outlet in the region of the throttle valve and provides a second bypass line. A second valve is mounted in the second bypass line or alternatively in the first start fuel line. The opening and closing of the second valve is controlled by the position of the choke valve. The carburetor further includes an accelerator pump for supplying additional fuel to the main fuel path downstream of the control valve during acceleration. This solution improves the operational range of the fuel supply. It is however costly and includes several additional components compared to e.g. U.S. Pat. No. 6,932,058.
U.S. Pat. No. 6,880,812 discloses a carburetor having two independent fuel supply systems, each including an electromagnetically driven control valve. A control system controls the opening and closing of the valves by using input from an engine speed sensor and a temperature sensor. Also this solution is costly and complex.
US 2009/0013951 shows a carburetor including a fuel supply system having two fuel paths for supplying fuel from a diaphragm controlled regulating to the intake channel of the carburetor. A main path supplies fuel to the intake channel during normal operations. A startup fuel supply passage has a solenoid valve to control the timing of startup fuel delivery. In this carburetor the fuel supply cannot be electronically controlled during normal operations since the solenoid valve only operates on the startup fuel supply passage. This is inadequate.